It is previously known that the corrosion of a metallic material can be studied using an electrochemical monitoring means such as Linear Polarization Resistance or Electrochemical Impedance. The theoretical basis for the use of electrochemical methods is usually founded in the Butler-Volmer relationship for a corroding material. The Butler-Volmer equation relates the current response of a system to an applied over-potential as a function of the corrosion current and the anodic and cathodic activation coefficients.
Another technique that has previously been used to study general corrosion processes is the Harmonic Distortion Analysis (HDA) method, which utilizes a low frequency sinusoidal voltage perturbation, and involves analysis of the current response in terms of the distortion due to the non-linearity of the current response. This technique involves further expansion of the Butler-Volmer equation, and facilitates analysis of the corrosion current, the anodic and cathodic activation coefficients and the so-called Stern-Geary constant.
While the above methods find wide usage for the study of general corrosion processes, those of skill in the appropriate arts have found that these techniques are generally unsuitable for the study of localized corrosion processes such as pitting corrosion.
For localized corrosion processes, a variety of other methods collectively termed “Electrochemical Noise” (EN) have been attempted. These techniques, which essentially involve analyzing the response of a corroding interface at the free corrosion potential with no applied perturbation, are used to evaluate the spontaneous changes in the corrosion processes, and are observed as current and potential fluctuations of a corroding specimen. Analysis of these potential and current noise signals may be in either the time or frequency domains.
During the measurement of low frequency impedance or when using the Harmonic Distortion technique, it is common practice to apply a high purity sine wave voltage perturbation and to measure the current at the fundamental frequency, and, in the case of Harmonic Distortion, to analyze the harmonic current content at integral multiples of the fundamental frequency. Since the applied voltage sinusoid is a periodic function, the current response at the fundamental frequency will be highly correlated with itself if the corrosion processes are stationary (in other words, when the processes do not change substantially during the period of measurements). Under these conditions, the self-correlation of the current response can be polled to validate the measurement, and also to serve as an indicator that further analysis (for example, harmonic distortion analysis) should be conducted to evaluate the corrosion processes for the anodic and cathodic activation coefficients. If, on the other hand, the corrosion processes are non-stationary, then the degree of self-correlation is substantially reduced, and will generally indicate a high probability for localized corrosion.
However, spontaneous changes occurring in the corrosion processes as a function of time can influence the current response to an applied potential perturbation, the extent of the influence depending on the relative magnitudes of the spontaneous variations in the corrosion potential, corrosion current and the activation coefficients. Thus, the stability of the response to an applied perturbation has often been viewed as a nuisance factor, with system and adventitious noise degrading the required response. Consequently, previously known electrochemical corrosion monitoring systems have been designed to reject noise, rather than analyze it in order to separate the deterministic response of the applied perturbation and any associated noise components.
There is, therefore, a longstanding and heretofore unmet need for an improved method and apparatus for continuously monitoring the corrosion of a working electrode, which provides simultaneous analysis for both the general corrosion and the localized corrosion processes, and which provides a reliable means for validation of the integrity of associated measurements.